Could the Milky Way have lost a lot of satellite galaxies?

The space around the Milky Way is not empty. It is teeming with dwarf galaxies, small, faint and low-mass, with a minimum of about 1,000 stars each. This is not unusual. We know from observations of other large galaxies that dwarf galaxies often congregate and can be captured in the gravitational field of the larger object. Astronomers have so far identified nearly 60 smaller galaxies within 1.4 million light-years of the Milky Way, although there is likely much more hidden in the dark.

According to new satellite data analysis Gaia, however, most of these galaxies are actually relatively young in the area, too new to orbit the Milky Way, at least not yet, the researchers think. The Gaia mission is an ongoing project to map the Milky Way with the utmost precision, including the three-dimensional positions, movements and speeds of the stars and objects in it.

Using measurements of these properties, the 'astrophysicist François Hammer of the Paris Observatory in France and his colleagues used the first piece of data released by Gaia to calculate the movements of 40 dwarf galaxies outside the Milky Way. Then they used parameters such as the three-dimensional velocity of each galaxy to calculate its orbital energy and angular momentum. The results were really intriguing, because they showed that most galaxies dwarfs presumed to be satellites of the Milky Way move much faster than objects known to be orbiting the Milky Way, such as the stars of Gaia-Enceladus and the spheroidal dwarf galaxy of Sagittarius.

The Milky Way has repeatedly cannibalized other galaxies throughout its long history. Gaia-Enceladus, was subsumed about 9 billion years ago. Its traces remain in a population of stars that orbit at relatively low energies. This discovery could alter our understanding of the interactions between normal galaxies and dwarf galaxies and the properties of dwarf galaxies, say the researchers. It is possible that some of the dwarf galaxies will be caught in the orbit of the Milky Way (although it is impossible to say which ones), but how long they will remain is an open question.

“The Milky Way is a large galaxy, so its tidal force is simply gigantic and it is very easy to destroy a dwarf galaxy after perhaps one or two passes, ”says Hammer. If a dwarf galaxy can survive longer, as was thought for dwarf galaxies in the Milky Way, something must hold them together, like higher concentrations of dark matter, the invisible glue that binds the Universe together.

The possibility that dwarf galaxies have a surprising amount of dark matter is already strongly suggested by the motions of their stars, which cannot be explained by the presence of normal matter alone. The new findings suggest that dark matter need not necessarily be included in our models of these galaxies.

However, there is a lot we don't know about the objects in and around the Milky Way, and there is absolutely none. no doubt that Gaia is changing our understanding of our little corner of the Universe.